1. Field of the Invention
The present invention concerns a method of forming wirings in a production step for semiconductor devices.
2. Description of the Prior Art
Along with recent improvement for the degree of integration in semiconductor integrated circuits, the size of devices have been reduced, as well as wirings connecting the devices have become finer and the multi-layered structure therefor has been developed further. The diameter for connection holes (contact holes, via holes) is also reduced along with the refinement of the wirings but reduction in the thickness of the inter-layer insulation film has a limit in view of the voltage withstanding and stray capacity. Therefore, the aspect ratio of the connection hole, that is, the depth-to-diameter ratio of the hole is naturally increased and, as a result, filling and flattening of the connection hole gives a significant problem.
As a method of attaining the filling and flattening, there has been a bias sputtering method of conducting sputtering while applying a negative DC bias voltage or RF bias voltage, a high temperature bias sputtering method of forming a film while heating a substrate, or a high temperature sputtering method using both of the above-mentioned methods together (for example, refer to Semiconductor World, Monthly Journal, General description by the present inventor, February, 1988, p 77).
In each of the methods, Al which is deposited on the substrate is at a high temperature, for example, of 425.degree. to 500.degree. C., and either vigorously migrates at the surface of the substrate or is entirely fluidizes to flow into the connection hole thereby improving the filling property to attain the flattening.
Further, a wiring of an Al alloy/Ti(O)N/Ti structure using TiN or TiON as a barrier metal has been often used recently for preventing Al from passing through a shallow diffusion layer in a contact hole.
However, even when the improving method for the filling and flattening of the connection hole at a high aspect ratio described above is used, the thickness of the Al film has to be increased in order to supply enough Al to fill the inside of the connection hole. For instance, in a case of a hole with a hole diameter of 0.8 um and a depth of 0.5 um, an Al film of 5000 .ANG. thickness forms a dent at the surface as shown in FIG. 4(a) and, accordingly, a thickness of greater than 6000 .ANG. is required for the Al film. The surface is flattened as shown in FIG. 4(b) in a case where the thickness of the Al film is 8000 .ANG..
In the drawing, 4 denotes an aluminum film (in particular, aluminum alloy film containing 1% Si), 2 denotes a titanium layer and 9 denotes an inter-layer film (SiO.sub.2). FIG. 4(a) and (b) are views prepared from reference photograph.
However, in a case of the Al film of 8000 .ANG. thickness, subsequent flattening for the inter-layer film is made difficult.
On the other hand, in a case of using the barrier metal, the crystal grain size of the Al film formed on the Ti(O)N film is decreased due to diffusion of Ti from the barrier metal film to bring about a problem that electro-migration resistance is reduced.